1. Field of the Invention
This invention relates generally to a high-voltage parallel relay system in vehicles, and, more particularly, to a high-voltage parallel relay system that adjusts the power flow through a parallel relay set when a fault is detected in one of the relays in the set.
2. Discussion of the Related Art
Electric and electric hybrid vehicles include relays that are often electromechanical devices that use an electromagnetic coil to move a contact system into a conducting (closed) position. The relays are often used in a charging circuit and in a propulsion circuit for conducting electricity to and from a high-voltage bus for a high-voltage battery.
The relays employed in electric vehicles are usually sized for the peak rate of the high-voltage battery. The peak rate is the peak charging rate for the charging circuit and the maximum load that the electric propulsion unit can place on the high-voltage battery for a given time. For example, an electric car may have a charging maximum of ten amps at 300 volts, so about 3 kilowatts. There will typically be a relay connected to the positive terminal of the battery and a relay connected to the negative terminal of the battery. When engineering work begins on a new electric or electric hybrid vehicle, the new vehicle is likely to have a different peak rate than the vehicles that have been designed previously, because of the rapidly evolving technology. Therefore, a new relay will need to be developed that can perform at the new peak rate. Furthermore, with the advent varying degrees of hybridization, for example, start and stop electric hybrid vehicles, there may be a plethora of high-voltage power bus needs all generating unique relay requirements. This is different from the low voltage side of the vehicle where the relays have been standardize for many years, for example, at 5, 10, 15, 20 or 40 amps at 12 volts.
There has been a desire in the automotive industry to commonize a set of high-voltage relays. However, because the technology keeps changing, it has not been practical to know what the design criteria would be for all of the high-voltage relays needed in the future. Engineers have discussed the use of lower-current high-voltage relays that are connected in parallel. Parallel relays have certain advantages, particularly, the amount of power that each relay needs to accommodate would be reduced, which allows the relay design to be simpler and less expensive. Also, by using relays electrically coupled in parallel, a new single high-voltage relay would not need to be designed for each new vehicle. Instead, once the peak power for the new vehicle is known, then the appropriate number of lower-current high-voltage relays could be wired in parallel for the new vehicle.
One problem associated with using multiple relays electrically coupled in parallel has been the condition that if one of the relays experiences a fault, then the entire parallel group of relays may fail to function as a result of its particular design. If the damaged relay is stuck open, then the remaining relays have to handle more power, which may tend to overheat the other relays possibly causing them to become stuck closed. If the damaged relay is stuck closed, which is the more likely scenario (being welded shut, due to the high voltages and elevated power levels), then the system loses the ability to control either the battery's charging or discharging (propulsion). Therefore, relay designs generally increase the structure and durability of one large relay, which tends to increase cost, and provides a single point of failure, which leaves the system susceptible to a ‘walk home’ condition.
In addition, there is a proliferation of charging requirements starting with various countries having different A/C power; for example, the United States uses 120 volts while Europe uses 220 volts. Different countries are putting different rules in place for faster charging of vehicles, like three phase charging. Therefore, if different relays were developed, such as one for each country, region, or power level stage for various vehicles, then there would be a need to develop, validate and maintain many different relays.
What is needed is a way to take advantage of parallel relays without the undesirable consequences of a complete failure if one of the lower-current, high-voltage relays in a parallel set fails.